1. Field of the Invention
The present invention relates to a semiconductor device and a method of manufacturing the device.
2. Description of the Related Art
A demand for high performance of LSI is increasing with sophistication of information in the society. Enhancement of the performance is achieved mainly by miniaturization of MOS transistors.
In the existing LSI, a gate insulating film of the MOS transistor has a thickness of about 1.5 nm. When the miniaturization simply progresses because of further enhancement of the performance, it is expected in International Technology Roadmap for Semiconductor (ITRS) that the thickness of the gate insulating film of the MOS transistor is about 0.7 nm around 2010.
However, when the gate insulating film becomes thin in this manner, a tunnel current flows through the gate insulating film at an operation time of the MOS transistor. This is called gate leak, and this is not preferable for the enhancement of the performance of the LSI.
To solve the problem, a technique has been researched which prevents the gate leak from being generated, even when the gate insulating film is thinned.
For example, a technique has been noted to use, as the gate insulating film, a material (high-k material) such as nitrogen-containing silicon oxide having a permittivity larger than that of silicon oxide which has heretofore been used well as the material of the gate insulating film.
For example, SiON containing a high concentration of nitrogen is one of candidates of the material (see, e.g., D. Matsushita et al., Symp. VLSI Tech., (2004) 172). This material has characteristics that the nitrogen concentration inside the gate insulating film is sufficiently high, although silicon oxide is used in an interface between the gate insulating film and a silicon substrate. Accordingly, there can be provided a gate insulating film having a high permittivity and satisfactory interface characteristic.
However, there is a problem caused by the high concentration of nitrogen in silicon oxide.
This is abnormal shift of flat band of a P-channel MOS transistor (see, e.g., Z. Wang et al., IEEE Electron Device Lett., 21 (2000) 170). That is, from a viewpoint of circuit design, when the flat band of the P-channel MOS transistor shifts, an impermissibly large problem is caused in the circuit characteristic. To solve the problem, it is necessary to give up the raising of the concentration of nitrogen in silicon oxide.